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Engine Discussion of preventative/corrective maintenance and other technical issues regarding your coach's engine.

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  #1  
Old 07-16-2008
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Somervillebus Somervillebus is offline
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Default Jake Brake, Transmission Sequence

Hello all, need some help with this issue. I arrived today in Gatlinburg Tn. for a couple of days off. Let me run this by you all ..

When driving @ 65 mph my tach reads about 1600 rpms. From that speed, if I activate the Jake Brake (high position) of course to slow down, the transmission selector automatically shifts the transmission into the 4th gear when I remove my foot off the throttle pedal, then the jake kicks in.

Once this occurs I get a warning light on the Pro-Driver (engine monitor) indicating an over rpm of around 2400 rpms .... so I'm thinking WOW! I then apply brakes to bring rpm down. This is also logged on the Pro-Driver as peak rpm (2400) WOW again!

This doesn't seem right to me at all. If I'm driving at say, 55-60 mph and apply the same as I stated above, no problems. I was under the understanding that you could apply the jake at any speed ????

Does everyones transmission with a similar set-up switch into 4th gear when applying the jake?? Is something out of sequence? Can anyone else shed some light on the situation here before I blow the top of my engine off? Or is this the way it operates. Its been like this since I've owned it.

No check engine, or transmission codes detected. Detroit, says call BB!

Many Thanks,

Slim Somerville
99 LXi
Kingston Springs, Tn.
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  #2  
Old 07-17-2008
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DD Series 60, right? The S-60 is redlined at 2100 RPM under load. It is redlined at 2400 RPM _not_ under load (using the Jake Brake). So, as long as you don't apply power while it's above 2100 RPM then you're still within specifications.

I note that when I step on the throttle, if the engine is above 2100 RPM the computer 'lags' the input until the transmission up-shifts so that the RPM is under 2100...

However, the downshift to 4th seems premature -- on my coach I've rarely observed the RPMs above about 2200 RPM when the Jake has been engaged. If speed is appropriate, it will shortly downshift to 5, then 4 as we slow down, usually at about 2200 RPM or so.

It might be appropriate to run this by tech-support at BB and/or take it in to be checked to ensure that the engine/transmission control computer is working correctly.

One other thought, the transmission has a "mode" selector (on or off), you might try turning it opposite of its current setting to see if that has any effect. The documentation I have on the transmission suggests that the mode selector "on" will make the jake less likely to kick on when operating on cruise control on a mild down slope.
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  #3  
Old 07-17-2008
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Default Jake Brake, Transmission Sequence

Pete,

Yes it is a series 60. I think the key now is load verses no load .. good points. I was able to talk to another Detroit/Allison shop foreman this morning. He stated once the jake is activated, and the throttle position sensor reads zero the shift indicator would show 4th gear, however, it does not go instantly into 4th, but rather works its way down to the 4th position once the Jake is activated. He says that if it did go into 4th @ 65-70 mph it would lock up ... so he doubts it working incorrectly.

He did not mention 5th showing on the selector display? However, he believes that if in-fact an over rpm occurred, it would send a check engine light indicating a logged code reflecting the situation. He also believes that it would'nt be a bad idea to have the system checked to make sure its all functioning properly.

I don't usually put the jake in the on position when using the cruise, I do understand it has its benefits. As far as the economy mode button, its usually on to prevent constant shifting. I will try it both ways to see if it makes a difference.

Thanks for the reply, I'm sure you can understand my concern with the engine being a 2100 rpm unit. So the load-no-load makes good sense.

Again Thanks,

Slim Somerville
99 LXi
Kingston Springs, Tn.
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  #4  
Old 07-17-2008
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On my shift display, I recall seeing the "5" pass by while slowing with the Jake brake. The display shows 2 numbers, the gear being used and the "target" gear (e.g. 4 or 3 when slowing as target and 6 or 5 as the "in" gear).

I generally leave my Jake switch in the "on" position as zero throttle is usually due to a desire to slow down. On the freeway, I usually leave the "low-med-high" switch in low -- it gives a rather mild braking effect that's helpful on gradual down slopes. I'll turn it off in rolling terrain, or where there are numerous over/under passes with associated grade changes, where the short downgrade is quickly followed by an upgrade -- then the loss of forward momentum simply wastes fuel for the next up slope.

I realize that a 99 LXi may have a different differential gearing than my '95, so that may account for my only seeing 2200 RPM (normally) while you may see the 2400 RPM from time to time. While 2400 RPM is the max, unloaded value, the computer control system is designed to avoid overspeed situations. The only way to overspeed the engine is to allow the coach to continue at a too-high speed downhill beyond the ability of the transmission to shift-up. If you're running down hill with the Jake, and the coach is gaining speed, it's time to (1) set the Jake to a higher level (the 6 cyl Series 60 uses 2, 4 or 6 cylinders for the braking effect) and (2) use the service brakes to bring the coach to a lower speed.

If the Jake is slowing you down, and downshifting, it sounds like it's doing its job. If it's slowing you more than you desire, then you should change the Low-Med-High switch accordingly.

At most, you may have a display issue if it isn't showing the current and target gear (check your Allison manual -- it may be a design difference). What you describe is consistent with my experience and the minor differences may simply reflect the manufacture date differences (and possible programing and/or gearing differences) between our two vehicles.
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  #5  
Old 07-17-2008
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Default Jake Brake, Transmission Sequence

It sounds as if all is functioning as described. I didn't think of the possibility of different gearing.I'll search a little further into that. I'm going to watch a little closer on the way home and monitor all functions. I certainly feel a little more at ease now know the S-60 rpm parameters.

Thanks

Slim Somerville
99 LXi
Kingston Springs, Tn.
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  #6  
Old 07-18-2008
davidmbrady
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Hi Slim,

On my '02 LXi, when traveling at 65mph and enabling the jake brake, the transmission will pre-select 4th and immediately downshift to 5th. At 65 mph in 6th I'm at 1600 rpm (give or take), when the jake is activated the immediate downshift to 5th puts rpm's at 1850. When the rpm's decrease to 1750 then the transmission downshifts to 4th and the tach shows 2350 rpm's avoiding excessive rpms.

It sounds like it's a good idea to have your system checked out. It appears that your LXi is skipping the initial slowing phase in 5th gear, and instead jumping right into 4th resulting in excessive engine rpm.

David Brady
'02 LXi, NC
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  #7  
Old 07-18-2008
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Slim,
THe only variation from what's been said so far is my BB manual shows my no-load max speed as 2300 rpm, not the previsouly stated 2400 rpm. I don't know if this is a simple difference of the years or a misprint in my manual. I haven't found anything that's from DD that shows this info yet. I'll be chatting with Ralph Fullenwider tomorrow, so he should be able to add all we want to know.
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  #8  
Old 07-18-2008
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It appears that S-60 max rpm spec's have changed over the years. My '02 DD S-60 Operator's Guide states a max power rpm of 2100 and a max no-load rpm of 2500 rpm.

David Brady
'02 LXi
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  #9  
Old 07-18-2008
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Gentlemen,

My manual shows load 2100 rpm no load 2225 no load. This is one of the reasons this discussion came about. Now it seems there are differences in the years involving the S-60. and, different manual specifications listed.

Other numbers to compare from my manual compared to others:

Bore: 5.12
stroke: 6.30
Displacement: 778 Cubic Inches
Compression Ratio: 15.0 to 1
Torque: 1450 Ft. Lbs. @ 1200 rpm
500 HP @ 2100 rpm
SAE Horsepower: 75.0 (not sure understand this number?)

Thanks

Slim Somerville
99 LXi
Kingston Springs, Tn.

Last edited by Somervillebus; 07-18-2008 at 08:32 PM. Reason: correction on torque
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  #10  
Old 07-18-2008
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Ok, my DD Engine Operator's Guide is noted "Rev. 05-95".

Displacement: 12.7 liters (or about 778 cubic inches)
Adv. HP 470 @ 2100 RPM
Min Idle 600 RPM
Max RPM NL 2225 RPM (The 2400 RPM figure comes from the BB owners manual that has many typos in it -- that may actually be referring to an 8v92!)

Elsewhere in the manual, it states that the Series 60 provides constant HP from 1800 through 2100 RPM "but fuel economy is not as efficient above 1800 RPM."

This reading of the Engine Operator's Guide explains why I've never seen the tachometer ever read above about 2200 RPM under no load conditions.

So, that leaves you with two possibilities. 1. the Tachometer is wrong. or 2. the DCEC system isn't set right. In either event, a visit to a DD service center and/or a BB service center is probably a wise precaution.
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